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Ake Malhammer | April 2005

The Thermal Interface Problem


the thermal interface problem

introduction

a few years ago solving an interface problem was just a matter of applying thermal grease. things are much more complicated today, mainly because the heat fluxes are increasing. the large selection of commercially available interface materials is a good illustration of this trend.

the interface problem has many facets that cannot all be discussed in this article. fortunately, there are experts such as the technical staff of suppliers to consult. the purpose of this article is, therefore, not to be complete, nor profound. it is rather to present an overview that hopefully could help some readers to be on the same page with the suppliers and other experts.

figure 1- low and high pressure contacts

low and high pressure contacts

there are no flat surfaces. pushing two surfaces together is like placing switzerland on top of austria. cavities cannot be avoided. with this image in mind, it is apparent that surface roughness and surface flatness are important parameters. another important parameter is the pressure. high pressures cause larger micro deformations than low pressures, figure 1. it should be noted that the scale on the y-axis is exaggerated. it is somewhat deceiving but it is the conventional way to illustrate surface roughness. a more proportional representation would look something like waves on a see surface swept by a breeze.

low pressure contacts are highly dependent of the fill material. for dry contacts it is air. for wet contacts it is usually a high viscosity fluid, for example grease. exclusively looking at the thermal conductivity, there is a factor 10 to gain by switching from air to a fluid. for thermally enhanced fillers, such as thermal grease, it is even higher. in practise, the gains are more modest, see: thermal interface materials. there are several reasons for this. the fill material has a thickness in itself that adds a thermal resistance and causes a loss of metal-to-metal contacts, (enhanced fill materials are often loaded with high conductive grains, which limits the minimum thickness). there is also a surface wetting problem, see below.

in this context, it is interesting to note that air sometimes can behave as a knudsen gas.  this appears when the mean free path, (molecular travelling length between collisions, ~60 nm at 1 atm), is of the same order as the size of the enclosing vessel. in practical terms, when the pressures is low or when the wall spacing is small, air might behave this way. interface cavities can actually be so tiny that this possibility must be considered. the result is, of course, a decrease in thermal conductivity of the joint.

figure 2- a method to detect the thermal resistance for a bottleneck

high pressure contacts are mainly dependent of the metal-to-metal contacts. each contact surfaces can be looked at as a bottleneck, (the flow lines form a bottleneck shape). figure 2 shows a method to detect that particular thermal resistance. several parameters are important here. surface roughness is one and the softness of the material is another. insightful research has revealed the physics of this case. with access to all parameters involved it seems possible to predict high pressure contact resistance reasonably well.

to get a perspective on this issue, it could be of interest to present some numbers. suppose a dry low pressure contact between two polished 30x30 mm aluminium blocks has been measured to 0.3 k/w. only considering air conduction, this corresponds to a ~7um air layer. the roughness for a polished surface is, however, of the order of ~1um, which indicates that it is the flatness of the surfaces and not the roughness that is the main parameter in this case, (a flatness that is ~10 times the roughness is realistic for “large” surfaces of the type discussed). considering the stochastic character of flatness, it is apparent that no conclusions can be based on one lab test only. the matter must be dealt with by statistical means.

taking the totally opposite approach and assuming that all heat is conducted by the metal, a bottleneck analyses results in 4% metal-to-metal surface contact. this is not realistic because it would involve a force of the order 10kn, (~1 metric ton). forces of that order can hardly be created with anything else than powerful bolt attachments.

figure 3- the wetting problem

interfaces within the interface

a much discussed problem is that most interface materials do not fill the cavities completely. as a result there are interfaces within the interface. the mathematical formulation is shown in figure 3. for a more profound discussion see: thermal interface materials: a brief review of design characteristics and materials. a fundamental conclusion is that the thermal resistance for an interface not only depends on the properties of the fill material but also on the surface structure.

there is, nevertheless, one liquid that is well known for its good wetting properties, silicon oil. even the tiniest drop on a surface extends to a palm sized stain within minutes. silicon oil is an excellent cavity filler. the drawback is that it also tends to creep out on adjacent surfaces. as a consequence, there is a dry out risk. a much more dramatic risk, however, is that silicon oil forms silicon oxide in presence of sparks. it is devastating for electrical contacts such as in relays. silicon oil was, therefore, banned in the telecom industry for many years. mechanical relays are recently being replaced by semi conductors, so things may have changed.

figure 4- the active interface surface is rarely the package top surface.

non-isothermal interfaces

just as there are no flat surfaces there are no isothermal surfaces either. it is a question of degree. almost isothermal surfaces can be created in the lab but they are rare in applications. it might be tempting to look at the top of a component as an isothermal surface. this is seldom the case. it all depends on what is underneath, figure 4. for all applications involving components with heat sinks it is important to look at the entire package. the 45 degree rule is often handy for a first estimate of the active contact surface. given the proportions in figure 4 it is only ~25 % of the total top surface. all applications are not that bad but large errors can evidently occur if this impact is neglected.

it should be emphasized that the 45 degree rule is an approximation intended for plates with one isothermal side. this is not quite the case for the example above but the rule is conservative. it works relatively well when the thermal resistance in the interface is of the same order as the thermal resistance in the component top. in the example above the corresponding range was ~2 and the error when applying the rule ~20%.  the error is larger for more extreme cases, for example components with highly conductive tops. the 45 degree rule is nevertheless recommended for over view purposes. that is, as a signal if the matter needs to be looked into more closely or not.

figure 5- yet another non-isothermal interface

figure 5 shows another non-isothermal interface problem. it is common in radio base stations. amplifier transistors are in this case screw fitted on a large heat sink. the heat fluxes are high, which induces large temperature gradients. there is a thermal expansion coefficient mismatch and the pressure is much higher below the screws than elsewhere. the result of it all is that the transistor foot forms a slight bow. it is needless to say that this effect is problematic. figure 6 shows how bad the situation can be.

figure 6- example of temperature profile below a transistor such as in figure 5.


what is a thermal contact resistance?

the general definition of a thermal resistance is very simple. it is the ratio of a temperature difference and a heat flow. this is not a problem for well conditioned laboratory cases but as shown above, it is often problematic in practical applications. the main dilemma is the temperature difference definition. it is easy to make all kinds of definitions, (a definition is by definition always correct), but it is very difficult to make them apply to all situations.

figure 7- the sandwich method.

the truth of the matter is that the thermal contact resistance concept rarely can be used for applications. an alternative approach is the sandwich method, figure 7. it consists of including parts of the surrounding structures in a thermal resistance definition. for a package it typically results in a rj-s value, (junction-to-heat sink). the basic idea is to push the temperature definitions away form the contact surface, where the temperature gradients are high, onto surfaces with more uniform temperatures. such surfaces do not exist, so there will still be uncertainties but on a much lower level. this method is good for hands-on measurements. the drawback is that the constituting thermal resistances cannot be separated. if something changes, for example the fill material, new measurements have to be made.

another approach is finite element calculations. the drawback in this case is the uncertainties in the input parameters. the thickness and the wetting properties of the fill material are difficult to determine. the result is, nevertheless, almost always a very good overview that is helpful for understanding the problems involved.

it can be concluded that not knowing what a thermal contact resistance is, it is hard to determine it with sufficient certainty. there are works around but none of them are as clear cut as a first superficial look at the problem may suggest.  


bout ake malhammar

 ake obtained his master of science degree in 1970 at kth, (royal institute of technology), stockholm. he then continued his studies and financed them with various heat transfer engineering activities such as deep freezing of hamburgers, nuclear power plant cooling and teaching. his ph.d. degree was awarded in 1986 with a thesis about frost growth on finned surfaces. since that year and until december 2000 he was employed at ericsson as a heat transfer expert. currently he is establishing himself as an independent consultant.

having one foot in the university world and the other in the industry, ake has dedicated himself to applying heat transfer theory to the requirements of the electronic industry. he has developed and considerably contributed to several front-end design methods, he holds several patents and he is regularly lecturing thermal design for electronics.

ake malhammar
frigus primore
http://www.frigprim.com/

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